Electrolyte additive for lithium rechargeable organic electrolyte battery

ABSTRACT

A large excess of lithium iodide in solution is used as an electrolyte  adive to provide overcharge protection for a lithium rechargeable organic electrolyte battery.

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto us of any royalty thereon.

The invention relates in general to a rechargeable lithium organicelectrolyte battery and in particular to an electrolyte additive forsuch a battery that provides overcharge protection.

BACKGROUND OF THE INVENTION

Rechargeable lithium-organic electrolyte batteries are being developedto provide low cost, high energy density power sources forcommunication, night vision and various other army applications.Typically, a rechargeable lithium organic electrolyte battery includes alithium anode, a cathode including compounds such as titanium disulfide,molybdenum oxide, molybdenum sulfide, vanadium oxide, vanadium sulfide,chromium oxide, etc and an electrolyte solution including an inorganiclithium salt such as lithium hexafluoroarsenate, lithium perchlorate,etc in organic solvents such as tetrahydrofuran, 2-methyltetrahydrofuran, etc.

One of the common problems of most lithium organic electrolyte batteriesis the oxidation of the solvent during the overcharging of these cellsresulting in the degradation of electrolyte solutions. It has beensuggested that the oxidation of organic solvents during the overchargingoperation could be prevented by using lithium iodide as an electrolyteadditive. Thus, during the overcharging operations, lithium iodide wouldbe preferentially oxidized first to lithium tri-iodide and eventually toiodine at lower potentials than the solvent oxidation potential and thuswould prevent the oxidation of the solvent. In laboratory experimentshowever, it was found that the liberated iodine itself reacted with theethereal solvents such as tetrahydrofuran in the presence of lithiumhexafluoroarsenate salt and caused its polymerization rendering thisapproach infeasible.

SUMMARY OF THE INVENTION

The general objective of this invention is to provide overchargeprotection for a lithium rechargeable organic electrolyte battery. Amore particular object of the invention is to effectively use anelectrolyte additive to provide overcharge protection for such abattery.

It has now been found that the aforementioned objects can be attainedusing a large excess of lithium iodide in the concentration range ofabout 0.5 to 1.0 molar.

In the presence of excess lithium iodide, the liberated iodine complexeswith lithium iodide to form lithium tri-iodide because of the highequilibrium constant of 10⁷ and thus does not react with thetetrahydrofuran solvent. Lithium tri-iodide is stable in these solutionsand will react with lithium metal to regenerate lithium iodide that willthen provide overcharge protection in the following cycles. The use ofexcess lithium iodide will thus provide overcharge protection inrechargeable lithium-organic electrolyte cells and increase the cyclelife of these cells.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In a typical cell configuration such as lithium/lithiumhexafluoroarsenate-tetrahydrofuran/titanium disulfide, the organicelectrolyte will contain a sufficient amount of lithium iodide additiveso that during the overcharging operations all of the lithium iodide isnot oxidized to iodine and there is always an excess of lithium iodidepresent in the solution to combine with the liberated iodine to formstable lithium tri-iodide. The solubility of lithium iodide intetrahydrofuran solutions is in excess of 1.0 molar and thus enough oflithium iodide can be added to the electrolyte to satisfy the conditionthat there always be an excess of lithium iodide present to complex withany iodine formed during the overcharging operations. Further, lithiumtri-iodide will react with lithium metal to regenerate lithium iodideand restore the cell to its original configuration and be available toprovide overcharge protection during the following cycles.

The invention is also applicable to lithium-organic electrolyte primarycells. In fact, the invention is applicable to all ambient temperaturelithium-organic electrolyte batteries.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described for obviousmodifications will occur to a person skilled in the art.

What is claimed is:
 1. In a lithium rechargeable organic electrolytebattery including lithium as the anode, a compound selected from thegroup consisting of titanium disulfide, molybdenum oxide, molybdenumsulfide, vanadium oxide, vanadium sulfide, and chromium oxide as thecathode, and a solution of an organic lithium salt in an organic solventas the electrolyte, the improvement of using a large excess of lithiumiodide in the solution as an electrolyte additive to provide overchargeprotection.
 2. A lithium rechargeable organic electrolyte batteryaccording to Claim 1 wherein the large excess of lithium iodide in thesolutions is about 0.5 to 1.0 molar.
 3. A lithium rechargeable organicelectrolyte battery according to claim 2 wherein the cathode is titaniumdisulfide.
 4. A lithium rechargeable organic electrolyte batteryaccording to claim 2 wherein the cathode is molybdenum oxide.
 5. Alithium rechargeable organic electrolyte battery according to claim 2wherein the cathode is molybdenum sulfide.
 6. A lithium rechargeableorganic electrolyte battery according to cClaim 2 wherein the cathode isvanadium oxide.
 7. A lithium rechargeable organic electrolyte batteryaccording to claim 2 wherein the cathode is vanadium sulfide.
 8. Alithium rechargeable organic electrolyte battery according to claim 2wherein the cathode is chromium oxide.
 9. A lithium rechargeable organicelectrolyte battery according to claim 3 wherein the electrolyte is asolution of lithium hexafluoroarsenate in tetrahydrofuran.
 10. A lithiumrechargeable organic electrolyte battery according to claim 4 whereinthe electrolyte is a solution of lithium hexafluoroarsenate intetrahydrofuran.
 11. A lithium rechargeable organic electrolyte batteryaccording to claim 5 wherein the electrolyte is a solution of lithium12. A lithium rechargeable organic electrolyte battery according toclaim 6 wherein the electrolyte is a solution of lithiumhexafluoroarsenate in tetrahydrofuran.
 13. A lithium rechargeableorganic electrolyte battery according to claim 7 wherein the electrolyteis a solution of lithium hexafluoroarsenate in tetrahydrofuran.
 14. Alithium rechargeable organic electrolyte battery according to claim 8wherein the electrolyte is a solution of lithium hexafluoroarsenate intetrahydrofuran.